Cognitive Radio from Hell: Flipping Attack on Direct-Sequence Spread Spectrum

In this paper, we introduce a strong adversarial attack, referred to as the flipping attack, on Direct-Sequence Spread Spectrum (DSSS) systems. In this attack, the attacker, which is appropriately positioned between the transmitter and the receiver, instantaneously flips the transmitted symbols in the air at 50% rate, thereby driving the channel capacity to zero. Unlike the traditional jamming attack, this attack, when perfectly executed, cannot be detected at the receiver using signal-to-noise-ratio measurements. However, this attack necessitates the attacker to perfectly know the realizations of all the channels in the model. We first introduce the consequences of the flipping attack on narrowband frequency-flat channels, and subsequently discuss its feasibility in wideband frequency-selective channels. From the legitimate users' perspective, we present a method to detect this attack and also propose heuristics to improve the error-performance under the attack. We emphasize that future cyber-physical systems that employ DSSS should design transceivers to detect the proposed flipping attack, and then apply appropriate countermeasures

High Rate Single-Symbol Decodable Precoded DSTBCs for Cooperative Networks

Distributed Orthogonal Space-Time Block Codes (DOSTBCs) achieving full diversity order and single-symbol ML decodability have been introduced recently for cooperative networks and an upper-bound on the maximal rate of such codes along with code constructions has been presented. In this report, we introduce a new class of Distributed STBCs called Semi-orthogonal Precoded Distributed Single-Symbol Decodable STBCs (S-PDSSDC) wherein, the source performs co-ordinate interleaving of information symbols appropriately before transmitting it to all the relays. It is shown that DOSTBCs are a special case of S-PDSSDCs. A special class of S-PDSSDCs having diagonal covariance matrix at the destination is studied and an upper bound on the maximal rate of such codes is derived. The bounds obtained are approximately twice larger than that of the DOSTBCs. A systematic construction of S-PDSSDCs is presented when the number of relays $K \geq 4$. The constructed codes are shown to achieve the upper-bound on the rate when $K$ is of the form 0 modulo 4 or 3 modulo 4. For the rest of the values of $K$, the constructed codes are shown to have rates higher than that of DOSTBCs. It is also shown that S-PDSSDCs cannot be constructed with any form of linear processing at the relays when the source doesn't perform co-ordinate interleaving of the information symbols.Comment: A technical report of DRDO-IISc Programme on Advanced Research in Mathematical Engineerin

A Novel Power Allocation Scheme for Two-User GMAC with Finite Input Constellations

Constellation Constrained (CC) capacity regions of two-user Gaussian Multiple Access Channels (GMAC) have been recently reported, wherein an appropriate angle of rotation between the constellations of the two users is shown to enlarge the CC capacity region. We refer to such a scheme as the Constellation Rotation (CR) scheme. In this paper, we propose a novel scheme called the Constellation Power Allocation (CPA) scheme, wherein the instantaneous transmit power of the two users are varied by maintaining their average power constraints. We show that the CPA scheme offers CC sum capacities equal (at low SNR values) or close (at high SNR values) to those offered by the CR scheme with reduced decoding complexity for QAM constellations. We study the robustness of the CPA scheme for random phase offsets in the channel and unequal average power constraints for the two users. With random phase offsets in the channel, we show that the CC sum capacity offered by the CPA scheme is more than the CR scheme at high SNR values. With unequal average power constraints, we show that the CPA scheme provides maximum gain when the power levels are close, and the advantage diminishes with the increase in the power difference.Comment: To appear in IEEE Transactions on Wireless Communications, 10 pages and 7 figure